Hyperacute rejection of transplanted organs and tissues is a well recognized phenomenon characterized usually by distinctive vascular changes (necrotizing arteritis, thrombi, hemorrhages, PMN infiltration) which resemble the Arthus phenomenon. It is associated with specific antiserum. Skin grafts rejected in this fashion either show an Arthus like picture, or failure to develop a blood supply (white grafts). White grafts and the closely related hemorrhagic ("red") grafts can also be elicited by a different immune mechanisms, which results in a picture differing histologically from that of the Arthus type: no vasculitis, no leukocytic infiltration, no thrombi. In contrast to the serum mediated (Arthus type) hyperacute rejection (SMHAR), it is cell mediated (CMHAR), and can be elicited right after a sensitizing cell injection; in fact, even shortly after the challenge graft has been transplanted. The white graft phase of this response starts right after sensitizing but lasts only a week or less. The purpose of the proposed study is the identification of the immune pathways of this type of hyperacute rejection (CMHAR), and specifically the cell type(s) involved; furthermore, a characterization of events at the target site, by ultrastructural and pharmacologic methods, and by analysis of the contribution made by the host to which effector cells are transferred; finally the relationship of CMHAR to the serum mediated form (SMHAR) in terms of immune pathways and kinetics. We hope that the information to be gained will help in a broader understanding of hyperacute rejection of transplanted organs, with possibly new avenues of prevention (donor selection) and therapy.